chore: ruff lint fixes and formatting across all source files

[korbonits]

Summary

Applies ruff check --fix and ruff format to all source files. No logic changes — behaviour is identical to master (and to #238, which this is based on).

Kept as a separate draft PR so reviewers can decide independently whether they want the formatting cleanup.

What changed

Rule	Count	Description
W293/W291	308	Trailing whitespace on blank/non-blank lines
I001	36	Unsorted / unformatted import blocks
F401	28	Unused imports (availability-check ones get # noqa: F401)
F541	8	f-strings without placeholders
UP009	8	Redundant # -*- coding: utf-8 -*- declarations
UP030	1	Explicit positional format index {0} → {}
F841	3	Unused local variables
E401/E701	4	Multiple imports / statements on one line
ruff format	21 files	Quote normalisation, line wrapping, indentation

Tooling config (in pyproject.toml from #238)

[tool.ruff]
line-length = 88
target-version = "py310"

[tool.ruff.lint]
select = ["E", "F", "W", "I", "UP"]
ignore = ["E501", "F403", "F405", "UP007"]

Test plan

All 65 unit tests from #238 continue to pass; the existing regression tests are unchanged.

uv sync --dev
uv run ruff check .   # clean
uv run pytest tests/  # 65 passed

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